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The invention relates to an arrangement for transferring information between the
secondary and the primary circuit of a transformer. The arrangement finds particular
utility in transferring control information needed in voltage stabilization across a
transformer used in a switched-mode supply.
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Switched-mode circuit solutions for providing supply voltage to electronic circuits
are very popular because of their relatively good efficiency. A switched-mode
circuit always needs an inductive component, usually a transformer, to store energy
in a magnetic field and to transfer it further to the load. The primary winding of the
transformer is connected to the feeding source of energy and the secondary winding
to the load. The energy supplied to the transformer by the primary winding must be
controlled according to the load. This requires that the voltage of the load be
monitored and a signal dependent of said voltage be transferred to the primary side
of the transformer to control the current in the primary winding. In order to
minimize the occurrence of malfunctions and to improve electrical safety, galvanic
isolation is provided between the secondary and the primary circuits of the
feedback.
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Arrangements are known from the prior art that include a separate component or
unit for the galvanic isolation mentioned above. One such known structure is depicted
in Fig. 1. It comprises a power transformer 10, switch unit 101, secondary
circuit 102, control unit 103 and an isolating unit 104. The transformer 10
comprises a primary winding wl, secondary winding w2 and a ferromagnetic core
depicted in Fig. 1 by vertical lines drawn between the windings. The primary
winding w1 belongs to a circuit that further includes said switch k and a source of
energy which has a certain source voltage V;. The switch k is used to "chop" the
current i1 in the primary winding. When the switch is closed, energy is stored in the
magnetic field of the transformer. When the switch is open, energy is discharged
from the magnetic field of the transformer to the secondary circuit 102. In the
simplified structure of Fig. 1 the secondary circuit comprises a rectifier diode D
followed by a filter capacitor C and a load connected to the terminals of said filter
capacitor. Load voltage Vo is sensed by the control unit 103 the output of which is
connected to the input of the isolating unit 104. The output signal vc of the isolating
unit is directed to the switch unit 101 controlled by it. The feedback is arranged
such that the load voltage Vo follows relatively faithfully a reference voltage
generated at the control unit.
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The input and output sides of the isolating unit 104 are galvanically isolated from
each other. The method of isolation is not specified in Fig. 1. The isolation may be
realized optically, for example, in which case the isolating unit comprises light
emitting and receiving components, among other things. The drawback of this
solution is that the feedback is relatively slow, which may result in stability problems
in voltage regulation. Inductive isolation is also known, in which case the isolating
unit comprises a transformer for that purpose. The drawback of this solution
is that a separate isolating transformer results in a considerable increase in production
costs.
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An object of the invention is to reduce said disadvantages associated with the prior
art. A structure according to the invention is characterized by what is specified in
the independent claim 1. Some advantageous embodiments of the invention are
specified in the other claims.
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The basic idea of the invention is as follows: Control information is transferred to
the primary circuit of a power transformer by means of auxiliary windings in the
same transformer. The auxiliary windings are arranged in pairs in the transformer
core branches in such a manner that the magnetic fluxes corresponding to the
control signal will not induce a voltage in the energy-transferring windings, and the
voltages induced by the magnetic flux corresponding to the energy transfer are
opposite both in the primary- and secondary-side winding pairs. Thus the transfer of
control signal and the transfer of energy will not disturb each other. The transformer
may be advantageously realized as a planar structure on a printed circuit board.
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An advantage of the invention is that it facilitates an arrangement of the transfer of
control information to the primary circuit of a power transformer at relatively low
production costs. This is made possible by the fact that the necessary auxiliary
windings have a small number of turns and can be arranged without an additional
stage in the production process. Another advantage of the invention is that it
facilitates fast feedback for a switched-mode structure, for example. A further
advantage of the invention is that the arrangement according to it is functionally
reliable.
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The invention is described in detail in the following. The description refers to the
accompanying drawings, in which
- Fig. 1 shows a prior-art arrangement in a switched-mode voltage source,
- Figs. 2a,b illustrate the functional principle of the arrangement according to the
invention,
- Fig. 3 shows an example of the arrangement according to the invention in a
switched-mode voltage source,
- Fig. 4 shows an example of a practical arrangement according to the invention,
- Fig. 5 shows another example of a practical arrangement according to the invention,
and
- Fig. 6 shows an example of a transformer core through which two control signals
may be transferred.
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Fig. 1 was already discussed in conjunction with the description of the prior art.
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Figs. 2a and b show a transformer constructed in accordance with the invention. The
core of the transformer has three branches. In the exemplary structure depicted in
the figure the first branch 21 is in the middle and has a cross-sectional area larger
than the other branches. Around the first branch there is a primary power winding
pw1 the terminals of which constitute port PP1, and a secondary power winding
pw2 the terminals of which constitute port PP2. The second branch 22 is shown to
be to the left of the first branch. Around the second branch there is a first primary
control winding cw1a and a first secondary control winding cw2a. The third branch
23 is shown to be to the right of the first branch. Around the third branch there is a
second primary control winding cw1b and a second secondary control winding
cw2b. The first primary control winding cwla and second primary control winding
cw1b are identical and connected in series. The terminals of the series connection
constitute port CP1. Likewise, the first secondary control winding cw2a and second
secondary control winding cw2b are identical and connected in series. The
terminals of this series connection constitute port CP2.
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For illustrative purposes, the operation of the structure described above is explained
with reference to two figures. Fig. 2a shows a situation in which a control signal Cin
is supplied to port CP1 while nothing is supplied to the other ports. Current ic1
corresponding to signal Cin flows through the first and second primary control
windings, generating a magnetic flux in the core of the transformer. The flux
generated in the first primary control winding cwla is divided from the second
branch 22 of the transformer core into the first and third branches. The flux
generated in the second primary control winding cw1b is divided from the third
branch 23 of the transformer core into the first and second branches. Said windings
are connected in series in such way that their fluxes are codirectional in the second
and third branches of the core. A flux c of a certain magnitude is generated in
them. On the other hand, the first and second secondary control windings around the
second and third branches are connected in series in such way that the voltages uc2
induced in them by the flux c are codirectional as observed from the terminals of
the series connection. Port CP2 thus gives a signal Cout that follows the variation of
the control signal Cin. The magnetic flux portions of the first and second primary
control windings directed to the first branch 21 of the core are opposite. Thus, no
voltages are induced in the primary power winding and secondary power winding,
i.e. both the voltage u1 of port PP1 and voltage u2 of port PP2 are zero. This means
that the energy of the control signal can be transferred from port CP1 to port CP2
without it being lost in ports PP2 and PP2. Neither will the transfer of the control
signal disturb the feeding of energy to the load.
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Fig. 2b shows a situation in which energy is supplied to port PP1 at power Pin while
nothing is supplied to the other ports. Current ip1 flowing through the primary power
winding pw1 causes in the transformer core a magnetic flux which is equally
divided from the first branch 21 to the second and third branches. There is in both
loops a flux p of a certain magnitude. The variation of the flux 2p of the first
branch induces in the secondary power winding pw2 a voltage which is used in
generating the load voltage. Energy is transferred to the secondary circuit at power
Pout. Now the fluxes in the second and third branches of the transformer core do not
form a flux circling the outer perimeter of the core, like in the case of Fig. 2a, but go
parallel, as observed in the direction of the branches. Therefore, the voltages
induced in the first and second primary control windings are opposite, as observed
from the terminals of the series connection of the windings in question. Voltage u3
of port CP1 is thus zero. Likewise, the voltages induced in the first and second
secondary control windings are opposite, as observed from the terminals of the
series connection of the windings in question, whereby voltage u4 of port CP2 is
zero, too. This means that the transfer of energy through the transformer to the load
will not disturb the transfer of control information between control ports CP1 and
CP2.
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Fig. 3 shows a structure corresponding to the switched-mode structure of Fig. 1. It
comprises a switch unit 101, secondary circuit 102, and a control unit 103 just as in
Fig. 1. What is different is that the control signal for the switch unit is now
transferred through the power transformer in accordance with the invention. The
structure and designators of the power transformer 20 correspond to Figs. 2a and b.
An external source of energy, which has a certain source voltage V;, is connected to
the primary power port PP1. The secondary power port PP2 is connected to the
secondary circuit 102. The output signal of the control unit 103 is connected to the
primary control port CP1, and the secondary control port CP2 is connected to the
switch unit 101. A separate isolating unit, such as block 104 in Fig. 1, is not needed
in this case.
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Fig. 4 shows a second example of the implementation of a transformer according to
the invention. In that implementation the core of the transformer comprises an E-shaped
part 48 and an I-shaped part 47. The primary power winding pwl and secondary
power winding pw2 are around the middle projection of the E part. The first
primary control winding cwla and first secondary control winding cw2a are around
the left end of the I part of the core. In the terms used in the description of Figs.
2a,b, said left end of the I part belongs to the second branch of the transformer core.
The second primary control winding cwlb and second secondary control winding
cw2b are around the right end of the I part of the core. In the terms used in the description
of Figs. 2a,b, said right end of the I part belongs to the third branch of the
transformer core. All windings are made before the E and I parts of the core are attached
to each other. Thus the increase caused by the control windings in the manufacturing
costs of the transformer is relatively small.
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Fig. 5 shows a third example of the implementation of a transformer according to
the invention. In this example the transformer is realized on a printed circuit board.
The circuit board 90 of Fig. 5 has three holes, such as 91, for taking the branches of
the transformer core through the board. The windings of the transformer are
conductive strips around said holes on the surface of the circuit board. In the
example of Fig. 5 there is on the upper surface of the circuit board 90 a spiral secondary
power winding pw2 around the middle hole. In addition, there is on the upper
surface, around the left hole 91, a single-turn first secondary control winding
cw2a and around the right hole a single-turn second secondary control winding
cw2b. These control windings are connected in series according to the invention
such that they are opposite in direction. Invisibly on the lower surface of the circuit
board there are in the corresponding fashion a primary power winding pwl, a first
primary control winding cwla and a second primary control winding cwlb.
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In the example of Fig. 5 the transformer core consists of an E-shaped part 58 and an
I-shaped part 57. For clarity, these are drawn pulled out from their mounting
position. The E-shaped part has dimensions such that its three projections match the
holes in the circuit board 90. The I-shaped part 57 is attached from the opposite side
of the circuit board to the projections of the E-shaped part so that in this case, too,
two loops are produced that are magnetically well conductive. The projections of
the E-shaped part are short, so the whole transformer structure is relatively flat.
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The printed circuit board onto which the transformer is assembled may naturally be
a multilayer board as well. Windings of the transformer may then be advantageously
positioned in the various intermediate layers.
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Above it was described some solutions according to the invention. The invention is
not limited to those solutions only. The shape of the transformer core may vary
greatly. It also may include more than three branches. One such core is the X core,
depicted in Fig. 6 from above and from the side. The X core comprises a center pole
and, symmetrically, two pairs of other poles. In addition, it includes an upper plate
66 and lower plate 67 that close the magnetic circuits. Power windings are placed
on the center pole 61. Windings for the transfer of one control signal can be positioned
in the pole pair 62, 63. In the other pole pair 64, 65 it is then possible to place
the windings needed for the transfer of another control signal. When the windings
are wired in accordance with the invention, the energy needed by the load and two
separate control signals can be transferred through the transformer without any one
of them disturbing the other two.
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Furthermore, the invention does not limit the materials used in the transformer, nor
its manufacturing method. The inventional idea may be applied in numerous ways
within the scope defined by the independent claim.